We have performed several tests of a channel model for the role of calcium in exocytosis. The model postulates that secretory vesicles contain both Ca-activated K channels and anion channels. When calcium enters the cell, it binds to sites on the Ca-activated K channels, allowing these channels to open. The opening of these channels and the presence of anion channels cause K and anions to enter the vesicles, thus increasing their osmotic pressure and causing an influx of water. For those vesicles situated very close to the cell plasma membrane, this could lead to fusion with the membrane and exocytosis of the vesicle contents. Evidence in support of the channel model was obtained by reconstituting Ca-activated K channels and anion channels from secretory vesicles of the bovine neurohypophysis into lipid bilayers. An interesting and somewhat surprising property of these Ca-activated K channels is that, in steady state, they open for only a narrow range of calcium concentrations - about 0.1-0.5 uM. The closing of these channels as the calcium concentration increases suggests a mechanism that could explain the inverse dependence of parathyroid hormone (PTH) secretion on calcium concentration. Patch clamp experiments on parathyroid cells are consistent with this mechanism: Ca-activated K channels close down as calcium concentration is increased in the submicromolar range.